1. Field of the Invention
The present invention relates to a magnetic recording medium in which a magnetic layer is formed by depositing a metal magnetic thin film on a nonmagnetic base by vacuum evaporation, i.e., oblique incident evaporation.
2. Description of the Related Art
Recently, as an information recording system is shifted from an analog recording system to a digital recording system, a demand for higher density recording of a magnetic recording medium as an information recording means is increased.
In order to realize a higher surface recording density by making a track narrower and making a line recording density higher, it is essential to obtain a higher output and a lower noise of a magnetic recording medium in a short-wavelength region.
An attempt of achieving a higher output of the magnetic recording medium largely depends upon a magnetic energy inherent in a ferromagnetic material forming a magnetic layer, and has reached a technical limit, which makes further improvement thereof difficult.
When a metal thin film is employed for a magnetic layer of the magnetic recording medium, a noise production mechanism lies in a microscopic region (a region of an electronic microscope) such as a crystal structure of a magnetic thin film, a magnetic interaction or the like, an attempt of achieving a lower noise of the magnetic recording medium lies in a field where future technical development is expected.
Specifically, the achievement of the lower noise is the most important technical problem in realization of higher recording density.
In the digital recording, since a noise largely influences an error rate in the form of a S/N ratio, the achievement of a lower noise as well as reduction of a data dropout is an important technical element for securing data reliability.
When a magnetic layer 24 formed of a metal magnetic thin film is formed on a nonmagnetic base 23 by vacuum evaporation, a column (so-called column-shaped crystal) 21 made of an evaporation material or its reactant is grown in a diagonal direction because of a structure of an evaporating apparatus as shown in FIG. 1.
The column 21 has a dense portion 21a and a sparse portion 21b. As the column 21 is grown more, the dense portion 21a becomes larger and a particle size d of the column 21 also becomes longer.
It is generally known that sizes of the column 21 and a crystal particle 22 in the column 21 influence generation of noises. In the magnetic recording medium having the magnetic layer formed of a metal magnetic thin film by evaporation, for achieving a lower noise, it is important to suppress growth of the column 21 and the crystal particle 22.
Which of the size of the column 21 or the crystal particle 22 in the column 21 influences more largely the generation of noises depends upon a wavelength of a recording signal to be used. In the range from 0.3 .mu.m to 1.0 .mu.m which is the shortest recording wavelength of a magnetic recording format which is now used, it is considered that the particle size d of the column 21 has more influence on the generation of the noises.
Since distribution of the particle size d of the column 21 also has influence on the noises, it is important to sharpen a curve shown in FIG. 2 and indicative of the distribution of the column particle diameter as much as possible.
Specifically, as shown in FIG. 2, it is necessary to make dispersion degree of the column particle size d smaller.